Voyager 2 Closes on Termination Shock

by Paul Gilster on November 29, 2007

When I use the term ‘interstellar mission,’ people assume I’m talking about a far future crewed mission to a star like Alpha Centauri or Epsilon Eridani. But the two Voyager spacecraft are on an interstellar mission of a sort, meaning they’re eventually going to leave the Solar System entirely and head into true interstellar space. Because the Voyagers’ power looks sound enough to keep sending data for another decade or more, we should thus get an interesting look at how our solar neighborhood differs from the medium that Sol and all the other stars in the Orion Arm swim in.

The termination shock is that place where the solar wind — charged particles flowing outward from the Sun — slows below the speed of sound. It should be a tricky and mutable place, there being no fixed boundary out there some eight billion or so miles from our star. Instead, the termination shock should vary depending on solar activity and other factors we may learn more about as we study the plasma, gas and dust outside. Thus Voyager gets the probable chance to pass through the shock more than once, as Haruichi Washimi (UC Riverside) notes:

“After it crosses this boundary, Voyager 2 will be in the outer heliosphere beyond which lies the interstellar medium and galactic space. Our simulations also show that the spacecraft will cross the termination shock again in the middle of 2008. This will happen because of the back and forth movement of the termination-shock boundary. This means Voyager 2 will experience multiple crossings of the termination shock. These crossings will come to an end after the spacecraft escapes into galactic space.”

Washimi’s simulations say that Voyager will cross the termination shock almost any time now, perhaps as late as early next year. He and his team are making their predictions based on geomagnetic disturbances caused by what’s happening on the Sun, their data drawing on what Voyager 2 has already passed along. Voyager 1 passed the termination shock in December of 2004. Up next for both craft: The heliopause, where the solar wind comes to a halt and interstellar space really begins.

The work of Washimi and team is scheduled to appear in the December 1 issue of The Astrophysical Journal.

Tacitus, it looks like a close call. NASA is estimating it will take ten to twenty years after reaching the termination shock to reach the heliopause. Whether either Voyager is still transmitting by then (or, at least, capable of being received) isn’t clear. Ten years sounds possible, but twenty is a reach.

What strikes me is the fact that both craft are expected to last until 2020, which is 43 years after launch. If we were able to get a probe to 1/10th light speed that is the time it would take for it to get to the nearest star. These two craft say a lot about our ability to one day build probes that could explore the nearer stars.

Yeah, except for the problem of colliding with even microscopic interstellar particles at that speed :) I believe the issue of armoring spacecraft is a major headache that has yet to be solved.

Anyway, any interstellar craft launched today has to get there much faster than that. I want to still be alive when the first images are returned, and I would be over 90 by then, and that’s pushing it! :)

Actually voyager gives me hope on that front too. It has already traveled 1/2700 of the distance needed to get to the centauri system (though not headed in that direction) and it still hasn’t been destroyed by collisions. Hopefully it means that a magnetic field approach will be enough to give a vessel a decent probability of surviving the journey. That anything over the size of a few atoms is rare enough that a vessel is unlikely to encounter it.

I can see three ways that we might hope to get a probe across interstellar distances without it being destroyed. Two of which would be:

1. Minor probes ahead of the main probe that uses a magnetic field to clear the path. They would also be able to detect larger masses that the main probe could then avoid. A number of those minor probes could be carried by the main probe. Some would be lost but it would leave the main probe intact.

2. Launch thousands of minor probes. The majority would be destroyed, with hopefully some getting through. Such probes would be able to act individually or as a group, even being able to assemble into a larger probe once they’ve reached their destination. Enabling them to assemble into a larger probe would not be a difficult task for a civilization that could actually launch something to another star, though some functions would not scale up. The main function of such an ability would be more power and a larger antenna so it can send data back to earth.

As for living to see images from such probes. I only hope I am alive to see humanity finally launch something like that. I have my doubts, but one can hope for a miracle.

That Voyager is getting to this area is fascinating. It would be great if we could launch a probe specifically for the purpose of examining this area. If we launched such a probe using Electric Ion propulsion would it get there quickly enough to be viable from a funding standpoint?

Kevin, with current ion technology the termination shock is a long haul indeed. There are interesting studies, though, for missions like Innovative Interstellar Explorer. Quoting one paper on this: “The Innovative Interstellar Explorer’s mission design used a combination of a high-energy launch using current launch technology, a Jupiter gravity assist, and electric propulsion powered by advanced radioisotope power systems to reach 200 AU.” You can read about this interesting concept, which was originally based on a solar sail, here:

Also, search Centauri Dreams for more references and discussion about this work. IIE is quite an interesting design; I’ll be talking about it again soon with reference to a paper on instrumentation needs for such missions.

San Francisco, Calif. – NASA’s Voyager 2 spacecraft has followed its twin Voyager 1 into the solar system’s final frontier, a vast region at the edge of our solar system where the solar wind runs up against the thin gas between the stars.

However, Voyager 2 took a different path, entering this region, called the heliosheath, on August 30, 2007. Because Voyager 2 crossed the heliosheath boundary, called the solar wind termination shock, about 10 billion miles away from Voyager 1 and almost a billion miles closer to the sun, it confirmed that our solar system is “squashed” or “dented”– that the bubble carved into interstellar space by the solar wind is not perfectly round. Where Voyager 2 made its crossing, the bubble is pushed in closer to the sun by the local interstellar magnetic field.

“Voyager 2 continues its journey of discovery, crossing the termination shock multiple times as it entered the outermost layer of the giant heliospheric bubble surrounding the Sun and joined Voyager 1 in the last leg of the race to interstellar space.” said Voyager Project Scientist Dr. Edward Stone of the California Institute of Technology, Pasadena, Calif.

The solar wind is a thin gas of electrically charged particles (plasma) blown into space by the sun. The solar wind blows in all directions, carving a bubble into interstellar space that extends past the orbit of Pluto. This bubble is called the heliosphere, and Voyager 1 was the first spacecraft to explore its outer layer, when it crossed into the heliosheath in December 2004. As Voyager 1 made this historic passage, it encountered the shock wave that surrounds our solar system called the solar wind termination shock, where the solar wind is abruptly slowed by pressure from the gas and magnetic field in interstellar space.

Even though Voyager 2 is the second spacecraft to cross the shock, it is scientifically exciting for a couple of reasons. The Voyager 2 spacecraft has a working Plasma Science instrument that can directly measure the velocity, density and temperature of the solar wind. This instrument is no longer working on Voyager 1 and estimates of the solar wind speed had to be made indirectly. Secondly, Voyager 1 may have had only a single shock crossing and it happened during a data gap. But Voyager 2 had at least five shock crossings over a couple of days (the shock “sloshes” back and forth like surf on a beach, allowing multiple crossings) and three of them are clearly in the data. They show us an unusual shock.

In a normal shock wave, fast-moving material slows down and forms a denser, hotter region as it encounters an obstacle. However, Voyager 2 found a much lower temperature beyond the shock than was predicted. This probably indicates that the energy is being transferred to cosmic ray particles that were accelerated to high speeds at the shock.

“The important new data describing the termination shock are still being pondered, but it is clear that Voyager has once again surprised us,” said Dr. Eric Christian, Voyager Program Scientist at NASA Headquarters, Washington.

The two Voyager spacecraft will be the only source of local observations of this distant but highly interesting region for years to come. But in the summer of 2008, NASA will be launching a mission specifically designed to globally image the termination shock and heliosheath remotely from Earth orbit. The Interstellar Boundary Explorer (IBEX), led by Dr. David McComas of the Southwest Research Institute in San Antonio, Texas, will use energetic neutral atoms (ENAs) to create all-sky maps at various energies of the interaction of the heliosphere with interstellar space. ENAs are formed when energetic electrically-charged particles “steal” an electron from another particle. Once neutral, they travel straight, unaffected by the solar magnetic field. IBEX will detect some of the particles that happen to be headed towards the Earth, and the number and energy of the particles coming from all different directions will tell us much more about the overall structure of the interaction between the heliosphere and interstellar space.

Results on the Voyager 2 shock crossing from the entire Voyager science team are being presented at the Fall 2007 meeting of the American Geophysical Union in San Francisco. The Voyagers were built by NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, Calif., which continues to operate both spacecraft.

Are there specifics known as to the eventual flyby of neighboring stars? I read somewhere that in about 40,000 years, Voyager 1 will be within 1.6 light years of a star in the constellation of Camelopardalis. Is it known which star that references?

Voyager is expected to arrive in the general area of Sirius in 296,000 years. Will V2’s transit path take it near a closer star before then?

Jeff, yes, here’s the deal on earlier spacecraft: Pioneer 10 is headed toward the red giant Aldebaran in the constellation Taurus, which it will reach in two million years. Pioneer 11 is moving in the direction of a star in the constellation Aquila, the Eagle, which it will reach in some four million years.

As to the Voyagers, in 40,000 years Voyager 1 will drift within 1.6 light years of a star known as AC+79 3888 in the constellation of Camelopardalis. Voyager 2 will reach the neighborhood of Sirius in 296,000 years, as you note. I know of no other star it will approach before this.

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In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last nine years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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